Chelly, I love how you think. Not just this post, I find myself agreeing with you much more often than not.
I believe the idea was that John was going to shower, so that he'd have no evidence left on him from doing the clean-up. After his shower, he would put the body in his 'large attache' and leave, where he would dump the body, then go withdraw the ransom money from the bank. That gives him an excuse to use later for why he left the house, were any neighbors to see him. He'd just be following the ransom demands. I believe that's why the note says they would call between 8 and 10 AM, then goes on to say if they monitor him getting the money early, they might call earlier.
Most banks I know of don't open before 8 AM at the earliest. Many don't open before 9. They supposedly would call between 8 and 10. How could he possibly get the money earlier? But I digress, I think that little blurb was put in there to allow him to get out of the house before many of their neighbors were up, and use the later excuse that he wasn't thinking, just rushed to the bank, then had to wait for them to open, which would give him plenty of time to dump the body somewhere, yet still be seen by any possible witnesses sitting in the bank parking lot before the bank opened.
I think Patsy blew the plan by calling 911 too early. I think she was supposed to call after John had left...perhaps even after she was sure the bank was open and he had the money. Her excuse for calling 911 would be that she panicked while JR was gone. The call then explains why JB is to be found dead later.
I think the problem was that she did actually panic- either from the thought of JB's body being dumped, or I've considered that whatever nerve-calming chemical she'd taken earlier was beginning to wear off and she was becoming hysterical. I've wondered if she hadn't ran to the laundry room and grabbed JB's favorite blanket and covered her body while JR was in the shower, then ran back upstairs to make the call- explaining why she was out of breath on the 911 call.
But this is all just part of my theory. Nothing concrete to back it up.